The disclosure relates to gas turbine engines. More particularly, the disclosure relates to fan tip clearance accommodation.
In turbofan engines, differences in the fan material versus fan case material may contribute to thermally-induced rub. Turbine engine fans and their cases experience differential thermal expansion across an operational range. For example, on the ground they are subject to a normal range in atmospheric temperatures (e.g., from 20 C to 40 C with an exemplary ambient temperature being 21 C). In flight, however, temperatures will typically decrease. Whereas other portions of the engine are subject to heating, the fan, and moreso, fan case temperatures may decrease at altitude (e.g., to an exemplary −60 C to −45 C). An exemplary temperature decrease from ground to altitude may be in excess of 50 C, more narrowly, 60-80 C or an exemplary 70 C.
With an exemplary metallic fan and non-metallic fan case (or structural portion thereof) the decrease in temperature will cause the fan to decrease in diameter more than the fan case (due to the fan having a higher coefficient of thermal expansion (CTE) than a structural portion of the fan case). When the inboard surface of the fan case moves radially with the structural case, a gap between fan blade tips and the fan case will increase, thereby potentially compromising performance.